Abstract

We report a study of magnetic susceptibility and electrical resistivity as a function of temperature and magnetic field in superconducting crystals of La 2-xCa 1+xCu 2O 6 with x = 0.10 and 0.15 and transition temperature T$$m\atop{c}$$ = 54 K (determined from the susceptibility). When an external magnetic field is applied perpendicular to the CuO 2 bilayers, the resistive superconducting transition measured with currents flowing perpendicular to the bilayers is substantially lower than that found with currents flowing parallel to the bilayers. Intriguingly, this anisotropic behavior is quite similar to that observed for the magnetic irreversibility points with the field applied either perpendicular or parallel to the bilayers. We discuss the results in the context of other studies that have found evidence for the decoupling of superconducting layers induced by a perpendicular magnetic field.

@article{osti_1433992,
title = {Evidence for magnetic-field-induced decoupling of superconducting bilayers in La2-xCa1+xCu2O6},
author = {Zhong, Ruidan and Schneeloch, J. A. and Chi, Hang and Li, Qiang and Gu, Genda and Tranquada, J. M.},
abstractNote = {We report a study of magnetic susceptibility and electrical resistivity as a function of temperature and magnetic field in superconducting crystals of La2-xCa1+xCu2O6 with x = 0.10 and 0.15 and transition temperature T$m\atop{c}$ = 54 K (determined from the susceptibility). When an external magnetic field is applied perpendicular to the CuO2 bilayers, the resistive superconducting transition measured with currents flowing perpendicular to the bilayers is substantially lower than that found with currents flowing parallel to the bilayers. Intriguingly, this anisotropic behavior is quite similar to that observed for the magnetic irreversibility points with the field applied either perpendicular or parallel to the bilayers. We discuss the results in the context of other studies that have found evidence for the decoupling of superconducting layers induced by a perpendicular magnetic field.},
doi = {10.1103/PhysRevB.97.134520},
journal = {Physical Review B},
number = 13,
volume = 97,
place = {United States},
year = {2018},
month = {4}
}

We explore the evolution of superconductivity in La 2 - x Ba x CuO 4 with x = 0.095 in magnetic fields of up to 35 T applied perpendicular to the CuO 2 planes. Previous work on this material has shown that perpendicular fields enhance both charge- and spin-stripe order within the planes. We present measurements of the resistivity parallel and perpendicular to the planes, as well as the Hall effect. Measurements of magnetic susceptibility for fields of up to 15 T applied both parallel and perpendicular to the planes provide complementary measures of the superconductivity. We show that fieldsmore » sufficient to destroy pair tunneling between the planes do not disrupt the superconducting correlations within the planes. In fact, we observe an onset of large-amplitude but phase-disordered superconductivity within the planes at approximately 30 K that is remarkably insensitive to field. With further cooling, we observe a phase-transition-like drop in the in-plane resistivity to an apparent state of superconductivity despite the lack of phase coherence between the layers. These observations raise interesting questions concerning the identification of the upper critical field, where pairing is destroyed, in underdoped cuprates.« less

Magnetoresistance measurements on a superconducting Nd{sub 2- x}Ce{sub x}CuO{sub 4-y} single crystal with {Tc} {approx} 7 K were made with magnetic fields H applied parallel (H {parallel} c) and perpendicular (H {perpendicular} c) to the tetragonal c-axis. For H {parallel} c, the resistive superconductive transition curves exhibit a double transition at low temperatures and high magnetic fields. This double resistive transition has a maximum near T {approx} 1.1 K, followed by an extremely sharp transition into the superconducting state. For applied fields of 1kOe, the resistivity first goes to zero, increases again to a finite value and then goes throughmore » a second transition. In this region, the resistivity exhibits non-ohmic behavior for low current densities and ohmic behavior for larger measuring currents. The resistivity for H {perpendicular} c does not exhibit any anomalous behavior or a maximum for 0 {le} H {le} 60 kOe and 4 {le} T {le} 7 K.« less

Magnetoresistance measurements on superconducting Nd[sub 2[minus][ital x]]Ce[sub [ital x]]CuO[sub 4[minus][ital y]] single crystals with [ital T][sub [ital c]] varying from 7 to 19 K were made with magnetic fields [ital H] applied parallel ([ital H][parallel][ital c]) and perpendicular ([ital H][perpendicular][ital c]) to the tetragonal c axis. For [ital H][parallel][ital c], the resistive superconductive transition curves gradually develop structure at low temperatures and high magnetic fields as [ital T][sub [ital c]] decreases. For the crystals with [ital T][sub [ital c]][approx lt]11 K, this structure evolves into a double resistive transition with a resistivity maximum near [ital T][approx]1.1 K, followed by anmore » extremely sharp transition into the superconducting state. In this region, the resistivity exhibits non-Ohmic behavior for low current densities and Ohmic behavior for larger measuring currents. The resistivity for [ital H][perpendicular][ital c] does not exhibit any anomalous behavior or a maximum. The irreversibility line inferred from these measurements is sample dependent.« less

We present an experimental study of the anisotropic resistivity of superconducting La{sub 2-x}Ba{sub x}CuO{sub 4} with x = 0.095 and transition temperature T{sub c}=32 K. In a magnetic field perpendicular to the CuO{sub 2} layers H{perpendicular}, we observe that the resistivity perpendicular to the layers {rho}{perpendicular} becomes finite at a temperature consistent with previous studies on very similar materials; however, the onset of finite parallel resistivity {rho}{parallel} occurs at a much higher temperature. This behavior contradicts conventional theory, which predicts that {rho}{perpendicular} and {rho}{parallel} should become finite at the same temperature. Voltage versus current measurements near the threshold of voltagemore » detectability indicate linear behavior perpendicular to the layers, becoming nonlinear at higher currents, while the behavior is nonlinear from the onset parallel to the layers. These results, in the presence of moderate H{perpendicular}, appear consistent with superconducting order parallel to the layers with voltage fluctuations between the layers due to thermal noise. In search of uncommon effects that might help to explain this behavior, we have performed diffraction measurements that provide evidence for H{perpendicular}-induced charge- and spin-stripe order. The field-induced decoupling of superconducting layers is similar to the decoupled phase observed previously in La{sub 2-x}Ba{sub x}CuO{sub 4} with x=1/8 in zero field.« less

{lambda}-(BETS){sub 2}GaBr{sub x}Cl{sub 4{minus}x} [BETS = bis(ethylenedithio)tetraselenafulvalene; 0 {le} x {le} 2] is a molecular superconductor with strongly correlated conduction electrons. The electrical transport properties of {lambda}-(BETS){sub 2}GaBr{sub x}-Cl{sub 4{minus}x} are drastically changed by varying the bromine content x or by applying pressure. At ambient pressure, the superconducting transition could be observed for x < 0.75. The pressure and x dependencies of {Tc} were examined. The M-H curve (M = magnetization; H = magnetic field) at 2 K indicated the almost perfect Meissner state of the superconducting phase of {lambda}-(BETS){sub 2}GaCl{sub 4}. The H{sub c1} is {approximately} 8 Oe formore » H{perpendicular} and 12 Oe for H{parallel}, where H{perpendicular} and H{parallel} are the magnetic fields perpendicular and parallel to the c axis, respectively. The magnetic susceptibility of {lambda}-(BETS){sub 2}GaBr{sub x}Cl{sub 4{minus}x} increases with decreasing temperature to {approximately} 60 K, below which the susceptibility becomes x-dependent and tends to be suppressed with increasing x. The isotropic decrease of the static susceptibility at lower temperature observed in the insulating system with x > 1.0 indicates the insulating ground state seems not to be suppressed with increasing x. The isotropic decrease of the static susceptibility at lower temperature observed in the insulating system with x > 1.0 indicates the insulating ground state seems not to be antiferromagnetic but probably nonmagnetic. The crystal structure determinations of a series of {lambda}-(BETS){sub 2}GaBr{sub x}Cl{sub 4{minus}x} and the calculations of the intermolecular overlap integrals of the highest occupied molecular orbital of BETS were made to elucidate a key factor of the superconducting transition mechanism. The x-dependence of intermolecular overlap integrals seems to suggest that the magnitude of the spin gap of the nonmagnetic insulating state tends to be diminished with decreasing x. There exists one intermolecular overlap integral exhibiting a large temperature and x-dependence, which seems to play a crucial role in determining the nature of the ground state.« less